I. Technical Field
This invention relates, in general, to non-contacting position sensors. More particularly, this invention relates to the magnetic configuration of non-contacting position sensors utilizing Hall effect devices, particularly those used in automotive environments.
II. Background Art
Electronic devices are an increasingly ubiquitous part of everyday life. Electronic devices and components are presently integrated in a large number of products, including products traditionally thought of as primarily mechanical in nature, such as automobiles. This trend is almost certain to continue. To successfully integrate electronic and mechanical components, some type of interface between the two technologies is required. Generally this interface is accomplished using devices such as sensors and actuators.
Position sensing is used to electronically monitor the position or movement of a mechanical component. The position sensor produces an electrical signal that varies as the position of the component in question varies. Electrical position sensors are an important part of innumerable products. For example, position sensors allow the status of various automotive parts to be monitored and controlled electronically.
A position sensor must be accurate, in that it must give an appropriate electrical signal based upon the position measured. If inaccurate, a position sensor will hinder the proper evaluation and control of the position of the component being monitored.
A position sensor must also be adequately precise in its measurement. The precision needed in measuring a position will obviously vary depending upon the particular circumstances of use. For some purposes only a rough indication of position is necessary, for instance, an indication of whether a valve is mostly open or mostly closed. In other applications more precise indication of position may be needed.
A position sensor must also be sufficiently durable for the environment in which it is placed. For example, a position sensor used on an automotive valve will experience almost constant movement while the automobile is in operation. Such a position sensor must be constructed of mechanical and electrical components which are assembled in such a manner as to allow it to remain sufficiently accurate and precise during its projected lifetime, despite considerable mechanical vibrations and thermal extremes and gradients.
In the past, position sensors were typically of the "contact" variety. A contacting position sensor requires physical contact between a signal generator and a sensing element to produce the electrical signal. Contacting position sensors typically consist of a potentiometer to produce electrical signals that vary as a function of the component's position. Contacting position sensors are generally accurate and precise. Unfortunately, the wear due to contact during movement of contacting position sensors has limited their durability. Also, the friction resulting from the contact can result in the sensor affecting the operation of the component. Further, water intrusion into a potentiometric sensor can disable the sensor.
One important advancement in sensor technology has been the development of non-contacting position sensors. As a general proposition, a non-contacting position sensor ("NPS") does not require physical contact between the signal generator and the sensing element. As presented here, an NPS utilizes magnets to generate magnetic fields that vary as a function of position and devices to detect varying magnetic fields to measure the position of the component to be monitored. Often, a Hall effect device is used to produce an electrical signal that is dependent upon the magnitude and polarity of the magnetic flux incident upon the device. The Hall effect device may be physically attached to the component to be monitored and move relative to the stationary magnets as the component moves. Conversely, the Hall effect device may be stationary with the magnets affixed to the component to be monitored. In either case, the position of the component to be monitored can be determined by the electrical signal produced by the Hall effect device.
The use of an NPS presents several distinct advantages over the use of the contacting position sensor. Because an NPS does not require physical contact between the signal generator and the sensing element, there is less physical wear during operation, resulting in greater durability of the sensor. The use of an NPS is also advantageous because the lack of any physical contact between the items being monitored and the sensor itself results in reduced drag upon the component by the sensor. Because the NPS does not rely upon an electrical contact, there is reduced susceptibility to electrical shorting caused by water intrusion.
While the use of an NPS presents several advantages, there are also several disadvantages that must be overcome in order for an NPS to be a satisfactory position sensor for many applications. Magnetic irregularities or imperfections may compromise the precision and accuracy of an NPS. The accuracy and precision of an NPS may also be affected by the numerous mechanical vibrations and perturbations likely be to experienced by the sensor. Because there is no physical contact between the item to be monitored and the sensor, it is possible for them to be knocked out of alignment by such vibrations and perturbations. A misalignment will result in the measured magnetic field at any particular location not being what it would be in the original alignment. Because the measured magnetic field will be different than that when properly aligned, the perceived position will be inaccurate. Linearity of magnetic field strength and the resulting signal is also a concern.
Some of these challenges to the use of an NPS have been addressed in existing devices, most notably the device of U.S. Pat. No. 5,712,561 issued to McCurley, et al and assigned to the CTS Corporation, herein incorporated by reference. There remains, however, a continuing need for a more precise determination of physical location of an item based upon the measured magnetic field at a location. Most particularly, a new type of non-contacting position sensor is needed which displays minimal deviations due to changes in temperature and maximum linearity of the magnetic field.